State of the art protective relays implement a thermal model that will trip if the estimated thermal state reaches a limit. For example, when starting a motor, the current is a maximum value that would be experienced in practice, and the thermal state increases at a maximum rate. Motor manufacturers typically provide parameters for maximum locked rotor time (e.g., starting time), from a cold state and from a hot state. These maximum time values are typically tens of seconds. Operating the motor at locked rotor (e.g., no rotation) for the specified time results in a maximum permitted temperature rise.
More advanced protective relays will learn the typical starting time, and trip the motor off line if the starting time greatly exceeds the learned value. This function will reduce the thermal stress on the motor, compared with allowing it to run the full allowed locked rotor time. However, this decision to trip is made after a duration corresponding to the typical starting time, e.g., after the motor should have reached a full running speed or running state. If the rotor is well and truly blocked, operating at a locked rotor condition for a duration at and beyond a typical starting time imposes unnecessary thermal stress on the motor.